Network Layer
1. OSI 7-Layer Model
Model Characteristics
Network standard specification established by ISO
Enables common network usage regardless of computer manufacturer
Each layer is independent, so it's not affected by other layers during data transmission process
Characteristics of Each Layer
Layer 1 (Physical Layer)
Controls physical connections between systems and converts electrical signals
e.g. Convert signals to digital or analog
Layer 2 (Data Link Layer)
Determines data transmission between network devices and physical addresses
e.g. Bridge/switch, MAC address
Layer 3 (Network Layer)
Responsible for data routing between actual networks and selects optimal paths
e.g. IP packet transmission
Layer 4 (Transport Layer)
Provides services to enable reliable data exchange between computers
e.g. TCP/UDP connections
Layer 5 (Session Layer)
Functions such as setting and releasing session connections, transmitting session messages
In other words, determines communication methods between computers
Layer 6 (Presentation Layer)
Encodes or decodes data passed to or received from application layer
e.g. Data conversion such as character codes, compression, encryption
Layer 7 (Application Layer)
Layer that ultimately provides interface with users
Application programs executed by users
e.g. File transfer, website viewing, etc.
Data Transmission Side
Data transmission side passes data from upper layers to lower layers
Each layer adds necessary information to data, and this information is called 'header' or 'trailer'
Adding headers is called 'encapsulation'
Data Receiving Side
Receives data passed through each layer from lower layers to upper layers
Passing data to upper layers and removing headers at each layer is called 'de-encapsulation'
After de-encapsulation, when reaching the final application layer, only the original data intended to be transmitted remains
2. TCP/IP 4-Layer Model
Model Characteristics
Model simplified and made practical for real-world use based on OSI model
Characteristics of Each Layer
Layer 1 (Network Interface Layer)
Corresponds to physical layer and data link layer of OSI layers
Uses MAC as physical address
LAN, packet network, etc.
Layer 2 (Internet Layer)
Corresponds to network layer of OSI layers
Responsible for transmitting IP packets between communication nodes and routing
e.g. IP, ARP, RARP
Layer 3 (Transport Layer)
Corresponds to transport layer of OSI layers
Controls connections between communication nodes and is responsible for reliable data transmission
e.g. TCP, UDP
Layer 4 (Application Layer)
Corresponds to session, presentation, and application layers of OSI layers
Used when implementing applications based on TCP/UDP
e.g. FTP, HTTP, SSH
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